Our invention comprises improvements in a printed circuit board of the type described in copending patent application Ser. No. 238,777, filed May 5, 1994 by P. A. Reddy, K. A. Salisbury, and J. D. Baker. That copending patent application is assigned to the assignee of our present invention.
Application Ser. No. 238,777 describes a printed circuit board comprising a population of electronic devices such as microprocessor chips, power transistor dies and other circuit components which tend to generate significant thermal energy when they are energized. It is usual practice in the manufacture of printed circuit board assemblies, such as the circuit board disclosed in the copending patent application, to prepare a substrate comprising a semi-rigid sheet by routing the outer boundaries. The sheet is drilled and cut to form openings for mounting circuit components.
The sheet may be formed of a glass-filled epoxy, paper, phenolic resin, etc., and a copper clad may be laminated to one or both surfaces. Copper lines are etched on the board to provide electrical connections between the pads, circuit components, terminal pins, etc. Solder paste can be dispensed at predetermined mounting locations of the electronic devices, bond pads, connectors, etc.; and electrical connections are established during transfer of the substrate with devices through an oven.
It is necessary to accommodate heat dissipation from certain electrical components, such as semiconductor dies, in order to avoid a temperature build up to unacceptable levels. Uncontrolled temperature build up may affect the reliability of the die. In the absence of a provision for heat dissipation, the reliability of the die can be maintained within acceptable quality standards only by reducing the operating power level.
It is known practice to mount silicon chips/semiconductor dies of an electronic device on a heat spreader if the electronic device is designed to operate at a relatively high power level, such as two watts or more. The heat spreader provides a thermal energy flow path between the electronic device, such as a power die, into a heat sink. The heat sink may be in the form of a housing assembled directly in contact with the printed wiring board in such a way that the heat spreader is capable of transferring thermal energy across the interface between the heat spreader and the heat sink.
The heat spreader typically is formed of a metal having high thermal conductivity, such as copper. If the electronic device is mounted directly on the heat spreader by soldering, for example, there will be a substantial localized differential in coefficient of thermal expansion at the interface of the die and the heat spreader. This increases the possibility of deterioration of the bond between the electronic device and the heat spreader.
Attempts have been made in prior art designs to compensate for a differential coefficient of thermal expansion of a semiconductor die, for example, and a copper heat spreader by molding a plastic encapsulation for the semiconductor die, whereby the semiconductor die is secured to the surface of the heat spreader. A stable bond between the semiconductor die base and the copper surface of the heat spreader thus is established at the base/copper interface. The process of encapsulating semiconductor dies in this fashion, however, requires additional processing steps and process equipment which results in an increase in the manufacturing cost of the devices and also the cost of printed wiring board assemblies.
Prior art U.S. Pat. No. 4,907,125 describes an attempt to avoid problems due to differential coefficients of thermal expansion between a metal plate and one side of a glass or ceramic body of a fuse or other thermally high-stressed component. A multi-layer plate that forms a carrier for electronic devices in the design of the 4,907,125 patent is comprised of an Invar layer sandwiched between two copper layers, one of the copper layers being in direct contact with the body of a fuse.
Invar is an alloy of iron and nickel and typically consists of 64% iron and 36% nickel. It is characterized by a very low coefficient of thermal expansion; i.e., 0-2.times.10.sup.-6 millimeters per millimeter per degree Centigrade. The coefficient of thermal expansion of copper, in contrast, is about 18-20.times.10.sup.-6 millimeters per millimeter per degree Centigrade. The entire substrate of the electronic device is bonded to one of the copper layers. Heat developed by the device, therefore, must be transferred through the poor thermally conductive substrate material and through an opening formed in the Invar layer. After passing through the metal layers, the heat also must pass through a relatively high thermal interfacial resistance at the interface of the surface of the substrate and the adjacent surface of the copper.
The use of a heat spreader consisting of copper and Invar layers is shown also in French Patent No. 2511193, dated Feb. 11, 1983 where a layer of Invar separates two copper layers and heat is transferred through the Invar metal posts. The two copper layers are not integral, and the electronic device is not mounted directly on the heat spreader layer.